GB2286594A - Golf balls and their covers - Google Patents

Abstract

A solid golf ball includes a cover which is mainly formed from a resin blend of (a) 50-90% by weight of a salt of an ethylene-unsaturated carboxylic acid copolymer having an unsaturated carboxylic acid content of 10 to 20% by weight wherein at least 20 mol% of the carboxyl group is neutralized with a metal ion and (b) 50-10% by weight of an ethylene-unsaturated carboxylate-unsaturated carboxylic acid terpolymer having an unsaturated carboxylate content of 12 to 45% by weight and an unsaturated carboxylic acid content of 5.5 to 10% by weight. The cover has a hardness of 60 to 80 on JIS C hardness scale. The golf ball has many advantages including a pleasant feel, ease of spin, ease of control, high repulsion and resistance to peeling.

Description

GOLF BALLS AND THEIR COVERS This specification relates to golf balls, and particularly to cover materials for them.

Prior Art As the cover material of two - piece golf balls and some wound golf balls, ionomer resins in the form of ethylene (meth)acrylic acid copolymers are widely used and accepted because of their long lasting impact resistance and cut resistance. Since the ionomer resins provide hard hitting feel and high hardness as compared with the balata rubber conventionally used as the cover material, golf balls with an ionomer resin cover are said difficult to impart a desired quantity of spin and inferior to control on iron shots.

For improvements in these respects, USP 4,884,814 or JP-A 308677/1989 proposes to blend an ionomer resin in the form of an ethylene (meth)acrylic acid copolymer having a certain spectrum of physical properties with a specific amount of an ethylene - (meth)acrylic acid (meth)acrylate terpolymer ionomer which is a relatively soft ionomer resin. The soft/hard ionomer blend is used as a golf ball cover.

This is a quite effective technique for improving the hitting feel and control of golf balls using a conventional ionomer resin in the form of an ethylene (meth)acrylic acid copolymer as the cover.

Nevertheless, the golf ball cover of the above-mentioned soft/hard ionomer blend has several problems. Since the cover is softer and has a lower hardness, the cover itself is less repulsive, leading to a golf ball having substantially reduced repulsion. Also the soft ionomer cover allows the ball to receive more spin by an iron shot, which means the increased frictional force between the club face and the cover. Then balls using a hard core such as two-piece solid golf balls have the likelihood that the ball surface be peeled as a consequence of iron shots because the cover surface can be scraped by grooves across the iron club face.

The aim here is to provide new and useful compositions for golf ball covers, as well as methods of preparation and use thereof.

In the light of the background discussion above, preferred aims are to provide good feel, ease of spin, ease of control, high repulsion and resistance to peeling.

Making efforts to develop a golf ball having a soft feel, good spin property, high repulsion, and improved resistance to peeling by iron shots, we have found that when a cover is based on a resin component consisting essentially of (a) 50 to 90% by weight of an ethylene - unsaturated carboxylic acid copolymer having an unsaturated carboxylic acid content of 10 to 20% by weight, at least 20 mol% of the carboxyl group being neutralized with a metal ion, and (b) 50 to 10% by weight of an ethylene - unsaturated carboxylate - unsaturated carboxylic acid terpolymer having an unsaturated carboxylate content of 12 to 45% by weight and an unsaturated carboxylic acid content of 5.5 to 10% by weight such that the cover has a hardness of 60 to 80 on JIS C scale, there is obtained a golf ball which is unlikely to be peeled on the surface and has a pleasant hitting feel, good spin property and satisfactory initial speed.

The golf balls proposed herein may be e.g.

solid golf balls having a solid core enclosed in a cover, for example, two- and three-piece golf balls and wound golf balls having a thread wound core enclosed in a cover.

The cover is composed mainly of a resin component consisting essentially of (a) 50 to 90% by weight of a salt of an ethylene unsaturated carboxylic acid copolymer having an unsaturated carboxylic acid content of 10 to 20% by weight wherein at least 20 mol% of the carboxyl group is neutralized with a metal ion and (b) 50 to 10% by weight of an ethylene - unsaturated carboxylate unsaturated carboxylic acid terpolymer having an unsaturated carboxylate content of 12 to 45% by weight and an unsaturated carboxylic acid content of 5.5 to 10% by weight. The cover has a hardness of 60 to 80 on JIS C scale.

Aspects of the invention are the compositions themselves, methods for their preparation, and their use in making golf ball covers and making golf balls, as well as the balls per se.

The cover of a golf ball is formed of a composition mainly comprising a resin component consisting essentially of (a) a salt of an ethylene unsaturated carboxylic acid copolymer and (b) an ethylene unsaturated carboxylate unsaturated carboxylic acid terpolymer.

Component (a) is a salt of an ethylene unsaturated carboxylic acid copolymer in which the carboxyl group is neutralized with a metal ion. The unsaturated carboxylic acid is preferably selected from aliphatic mono- carboxylic acids having 3 to 8 carbon atoms, especially acrylic acid and methacrylic acid.

The copolymer should have an unsaturated carboxylic acid content of 10 to 20% by weight.

With an unsaturated carboxylic acid content substantially less than 10% by weight, the resulting cover has low impact resilience. With an unsaturated carboxylic acid content substantially over 20% by weight, the resulting cover becomes hygroscopic and lowers its performance.

The salts of ethylene - unsaturated carboxylic acid copolymer include sodium, lithium, magnesium and zinc salts. From the standpoints of repulsion and impact durability, it is recommended to combine a monovalent salt (Li or Na salt) with a divalent salt (Mg or Zn salt), especially in a weight ratio of from 25:75 to 75:25. The degree of neutralization with the metal ion (which is the proportion of hydrogen atoms of carboxyl groups in the copolymer replaced by metal ions) is at least 20 mol%, preferably at least 25 mol%. With a degree of neutralization substantially below 20 mol%, impact resilience and resistance to peeling are low.

Typical are ethylene (meth)acrylic acid copolymer salts which are commercially available as HimilanX from Mitsui-DuPont Polychemical K.K., ionomers of the corresponding grade from E.I. DuPont de Nemours Co., and "Iotekel (ethylene - acrylic acid copolymer ionomer) from Exxon.

Component (b) is an ethylene-unsaturated carboxylate - unsaturated carboxylic acid terpolymer. The unsaturated carboxylic acid is preferably selected from carboxylic acids having 3 to 10 carbon atoms, especially unsubstituted monocarboxylic acids such as acrylic acid and methacrylic acid. The unsaturated carboxylate is generally an ester, preferably selected from esters of unsaturated carboxylic acid having 3 to 8 carbon atoms (e.g., acrylic and methacrylic acid) with alcohol having 1 to 9 carbon atoms.

Exemplary esters include methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, and isobutyl methacrylate.

The terpolymer may be either a random or a graft copolymer. Among the graft copolymers, preference is given to ethylene (meth)acrylate random copolymers having an unsaturated carboxylic acid graft polymerized thereto.

The terpolymer should have an unsaturated carboxylate content of 12 to 45% by weight, preferably 18 to 25% by weight and an unsaturated carboxylic acid content of 5.5 to 10% by weight, preferably 6 to 9% by weight. If the carboxylate content is substantially less than 12% by weight or if the carboxylic acid content is substantially more than 10% by weight, the resulting cover is less flexible and has low impact resilience. If the carboxylate content is substantially more than 45% by weight or if the carboxylic acid content is substantially less than 5.5% by weight, component (b) becomes less compatible with component (a) and the resulting cover has a lower resistance to peeling. If the unsaturated carboxylic acid content is less than 5.5% by weight, the resulting terpolymer is usually less adhesive to urethane paint which is used as an overcoat.An unsaturated carboxylic acid content of at least 5.5% by weight is therefore preferred too.

As mentioned, the cover of the golf ball is formed from a composition mainly comprising a resin component consisting essentially of 50 to 90% by weight of component (a) and 50 to 10% by weight of component (b), and such that the cover has a hardness of 60 to 80 on JIS C hardness scale. By blending components (a) and (b) in this range, there can be formed a cover which is soft and promotes ball spin, yet resistant to peeling. If the content of (a) is substantially less than 50% by weight or the cover's hardness substantially less than 60, the cover is too soft to provide the ball with sufficient repulsion. If the content of (b) is substantially less than 10% by weight or the cover's hardness substantially over 80, hitting feel and spin property become poor.

In general, a cover can be prepared by optionally adding to the resin compound consisting of components (a) and (b) dyes, pigments (e.g., titanium dioxide, zinc oxide and barium sulfate), UV absorbers, anti-oxidants, and dispersing aids (e.g., metal soaps), and mixing these components in a conventional mixing means, for example, a closed kneading machine (Banbury mixer and kneader) and extruder, followed by shaping in a conventional manner.

The core which is enclosed in the cover is not particularly limited and is selected from solid cores for solid golf balls such as two and three-piece golf balls and thread-wound cores.

The covers provided herein exert their function best when combined with solid cores having high repulsion.

The solid core of solid golf balls is preferably formed of a composition comprising a base rubber and a metal salt of an unsaturated carboxylic acid. The preferred base rubber used herein is cis - 1,4 - polybutadiene rubber having at least 90% of a cis structure because of high repulsion. The cis - 1,4 - polybutadiene rubber may be optionally blended with a suitable amount of natural rubber and polyisoprene rubber although the base rubber should preferably contain at least 80% by weight of high cis- 1,4 - polybutadiene rubber. If the content of cis - 1,4 - polybutadiene rubber is less than 80% by weight, the base rubber would not take full advantage of the high repulsion of cis-1,4- polybutadiene rubber.The metal salt of an unsaturated carboxylic acid is blended as a co- crosslinking agent, preferably in an amount of about 25 to 40 parts by weight per 100 parts by weight of the base rubber. A typical salt is zinc acrylate. A peroxide is also added to the core composition. Exemplary peroxides are dicumyl peroxide, t - butylperoxybenzoate, dit - butylperoxide, and 1,1 - bis(t - butylperoxy) 3,3,5 - trimethylcyclohexane. The peroxide is preferably blended in an amount of about 0.5 to 3 parts, especially about 0.8 to 2 parts by weight per 100 parts by weight of the base rubber.

In the core composition, one may blend other components e.g. as conventionally used in the core of two piece solid golf balls such as zinc oxide, anti-oxidant and barium sulfate, if desired. The core composition is kneaded in a Banbury mixer or kneader and molded and cured into a spherical core by hot pressure vulcanization e.g. in a conventional manner.

The core as cured preferably has a sufficient hardness to yield a distortion of 2.3 to 3.3 mm when a load of 100 kg is applied to the core. With a core distortion of less than 2.3 mm, a ball obtained by enclosing the core in a cover would be hard and give an unpleasant feel on hitting. A core distortion of more than 3.3 mm would lead to low repulsion.

A golf ball may be prepared by a conventional molding technique, for example, by preliminarily molding half cups from the cover material, interposing a core between the half cups and effecting heat compression molding or by directly enclosing a core with the cover material by injection molding. At the end of such molding, the cover on the surface is subject to finishing steps including buffing, lacquering and stamping.

In the resulting golf ball the cover preferably has a radial thickness of up to 2.2 mm, more preferably 1.2 to 2.1 mm when the ball is a solid golf ball using a solid core as mentioned above. The cover preferably has a radial thickness of up to 2.0 mm, more preferably 1.0 to 1.7 mm when a wound core is used. The ball would tend to lose a degree of repulsion if the cover thickness exceeds the above defined range or upper limit.

EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation.

Examples 1 - 5 and Comparative Examples 1 - 6 For the manufacture of a large size twopiece golf ball, a core having a diameter of 38.5 mm was prepared by furnishing a rubber composition of the following formulation and effecting vulcanization at 155"C for 18 minutes.

Rubber composition (Core) Parts by weight JSR BRO1 100 Zinc acrylate 33 Zinc oxide 9.7 Barium sulfate 9.7 Anti-oxidant (Nocrack NS-6 ) 0.2 Dicumyl peroxide 1.0 Next, a cover composition was prepared by using resin components as shown in Table 1, blending them in the proportion shown in Table 2, and adding 5 parts by weight of titanium dioxide and 0.8 parts by weight of magnesium stearate to 100 parts by weight of the resin components combined. By an injection molding technique, the cover composition was molded over the core to form a cover of 2.0 mm thickness thereon. The cover was coated with a urethane base clear lacquer, completing a golf ball.

The two piece golf balls were evaluated for spin, peeling, hitting feel and initial speed by the following tests.

(1) Hardness Hardness is expressed by a distortion (mm) of a ball under a load of 100 kg. Higher values indicate softer balls.

(2) Hitting feel Professional golfers hit balls with the driver (w#l) and 7th iron A hitting feel was evaluated according to the following criterion.

significantly softer than the feel of conventional two-piece golf balls n: slightly softer than the feel of conventional two - piece golf balls X : equal to the feel of conventional two - piece golf balls (3) Spin With a sand wedge mounted on a swing robot, a ball was hit at a head speed of 19 m/s.

The motion of the ball immediately after impact was analyzed using high-speed photographs, determining a degree of backspin (rpm).

(4) Initial speed An initial speed (m/s) was measured by an initial speed meter of the same type as the authorized organization R & (USGA).

(5) Scuff test A ball was kept at a temperature of 23 C.

Three commercial pitching wedges were mounted on a robot machine. The ball was hit at three positions by the three pitching wedges, once at each position, at a head speed of 37 m/s. The three hit areas were visually observed.

Evaluation was made according to the following criterion. Each result is an average of four balls tested.

Point Remarks 10: no change on the ball surface 9: intermediate points 10 and 8 8: very slight, substantially unperceivable club face dent 7: intermediate points 8 and 6 6: substantial club face dent, but no peeling from the cover surface 5: intermediate points 6 and 4 4: perceivably peeled surface 3: intermediate points 4 and 2 2: peeled surface with some cracks 1: outstanding cracks (6) Lacquer adhesion A ball mill was charged with a golf ball, sand (as used in a golf bunker) and water and operated. The ball was taken out of the mill and washed and the state of the lacquer coating was observed.

O: no change of lacquer coating x: lacquer coating stripped The results are shown in Table 2.

Table 1

Unsaturated Degree of Resin carboxylic Ester Composition neutralization component acid content content (mol %) (wt%) Copolymer salt ethylene - methacrylic acid (85/15 Ionomer A wt%) copolymer neutralized with 50 15 Li ethylene - methacrylic acid (85/15 Ionomer B wt%)copolymer deutralized with 29 15 Na ethylene - methacrylic acid (85/15 Ionomer C wt% copolymer neutralized with 27 15 Mg ethylene - methacrylic acid (85/15 Ionomer D wt%) copolymer neutralized with 58 15 Zn Terpolymer salt ethylene - methacrylic acid Ionomer E isobutyl acrylate terpolymer 70 8 20 neutralized with Na ethylene - methacrylic acid Terpolymer F unsaturated carboxylate - 8 20 terpolymer ethylene - methacrylic acid Terpolymer G unsaturated carboxylate - 4 20 terpolymer Table 2

~ o Example 1 2 3 4 5 ç 2 3 4 5 6 Resin O X lonomer A 30 35 O O n l O lonomer B 30 35 20 50 Wcr C 30 35 20 47 20 25 30 Wa ~ o o o ~ D 30 35 CO O m E 50 o o o F razz 30 40 6 60 60 Terpolymer G 40 Resin Oc7 properties JIS C hardness 75 79 75 79 N X 58 59 78 76 90 Golf ball O ~ Hardness 2.58 oo 2.56 CD O cu feel 0 0 0 0 0 x 0 0 0 0 x Spin 6500 Irl O 0 < 6050 6550 4510 co c speed 76.6 77.0 76.7 77.1 76.8 77.3 75.7 75.5 76.3 76.7 77.4 Scuff 8 7 8 8 8 7 3 3 7 5 7 Lacquer | n w O o k o ~ O 0 x 0 li3 ~ N o o N n n n b un O sn > > O < n O O O un U) U) O O t0 OD O 'O X CD Az C Q. O r r E < m U n X g E O < C W O U O W1 A h r1 H ~ = 1 :: 1 c 3 o o o o oz A c U Q r $ C fi W a H H H H H Eq Eq H O X SC U] H U] a Thus the golf balls embodying our proposals gave in combination a pleasant feel, ease of spin, ease of control, high repulsion and resistance to peeling.

Japanese Patent Application No. 6-36521 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims (7)

CLAIMS:

1. A solid golf ball comprising a cover based on a resin component consisting essentially of (a) 50 to 90% by weight of a salt of an ethylene - unsaturated carboxylic acid copolymer having an unsaturated carboxylic acid content of 10 to 20% by weight wherein at least 20 mol% of the carboxyl group is neutralized with a metal ion and (b) 50 to 10% by weight of an ethylene unsaturated carboxylate - unsaturated carboxylic acid terpolymer having an unsaturated carboxylate content of 12 to 45% by weight and an unsaturated carboxylic acid content of 5.5 to 10% by weight, said cover having a hardness of 60 to 80 on JIS C hardness scale.

2. A solid golf ball of claim 1 wherein the unsaturated carboxylic acid of component (a) has 3 to 8 carbon atoms, and the unsaturated carboxylic acid of component (b) has 3 to 10 carbon atoms and the unsaturated carboxylate is an ester of an unsaturated carboxylic acid having 3 to 8 carbon atoms with an alcohol having 1 to 9 carbon atoms.

3. A solid golf ball of claim 2 wherein the unsaturated carboxylic acid of component (a) is acrylic acid or methacrylic, and the unsaturated carboxylic acid of component (b) is acrylic acid or methacrylic acid and the unsaturated carboxylate is methyl(meth)acrylate, ethyl (meth) acrylate, isobutyl(meth)acrylate, n butyl(meth)acrylate or 2 - ethylhexyl(meth) acrylate.

4. A solid golf ball of any one of claims 1 to 3 wherein the salt of ethylene unsaturated carboxylic acid copolymer is a mixture of a monovalent salt and a divalent salt in a weight ratio of from 25:75 to 75:25.

5. A solid golf ball of claim 1 which is a two- or three-piece golf ball having a solid core enclosed in a cover.

6. A solid golf ball of claim 1 which is a wound golf ball having a thread wound core enclosed in a cover.

7. A golf ball substantially as described herein with reference to any of the Examples (but not the Comparative Examples).